I also encountered this error. For me the solution was that I had more columns of data in my data.dat file than I had specified in the "NAMES =" line. To remedy the problem I opened my datafile in SPSS, and compared the number of rows of variables to the total number of variables. Turns out, I had missed one in the NAMES list, and so MPLUS assumed that my final variable was categorical, when it was actually continuous. Adding the missing variable to the names list fixed the problem, so that MPLUS correctly saw my final column of data, which was there all along but I had not been telling MPLUS that it was there.